1. Field of the Invention
The present invention relates to television receivers and in particular to means or circuits for selecting a channel of a varactor type.
2. Description of the Prior Art
Varactor-type tuning circuits are well-known in the art and particularly have been adopted for tuning tuners including both UHF and VHF sections. Such tuning circuits incorporate a varactor diode whose capacitance is dependent upon a tuning voltage applied thereto. The amplitude of the tuning voltage is determined selectively to set the channel to which the tuner is to be tuned.
There are known methods of developing such tuning voltages, including the so-called automatic tuning in which a sweep is stopped when a predetermined intermediate-frequency signal is produced by applying voltages to the varactor diode from a voltage sweep circuit. Although this method makes the digital display of selected channels possible, it is disadvantageous in being unstable since the sweep can stop due to voice carrier waves and since a particular one of the receiving channels can change to another on account of interruption or weakness of the incoming electric waves.
Channel selecting apparatus has therefore been contrived in which two local oscillators, each adapted to operate with the frequency swept by sweep voltages, are provided. The two local oscillators are so set to alternately conduct the sweep initiation and the sweep termination; the frequency interval between the sweep initiation and the corresponding sweep termination is made equal to the channel frequency interval. The alternate sweep operations are started from the carrier frequency of the lowest frequency channel of a broadcasting band, and the number of times of the sweep initiations or terminations is counted to detect that the aforesaid operations have been carried out by a desired number of times. The sweep of the concerned one of the local oscillators is stopped upon detection, and then the sweep voltage at the stop is stored, to receive the desired broadcast. Such a varactor tuning circuit is described in U.S. Pat. No. 3,822,405.
Alternatively, the tuning voltage may be derived from a synthesizer circuit including a keyboard which the operator sets in accordance with the desired channel, to provide a signal to be stored within a channel memory. Illustratively, the output of the channel memory indicative of the channel to be tuned may be displayed upon a suitable display and also applied to a variable, programmable divider. The programmable divider selectively divides the output of a local oscillator within the varactor tuner to be compared with the output of a reference oscillator. A phase comparator mixes the reference output oscillator output and the output of the programmable divider to provide a variable, tuning voltage to be applied to the varactor-type tuner. One of the disadvantages of such channel-selecting circuitry is that the programmable divider uses a series of counters to achieve the division of the local oscillator output, which typically is of a high frequency. In the division process, relatively high-frequency radiation emanates from the programmable divider. Typically, where UHF and VHF signals are being tuned, these frequencies are in the order of about 50 MHz to about 920 MHz. Radiation of such frequencies is sensed by the receiver's antenna to impose extraneous signals upon the video signal, which cause distortion of the displayed image. In a color receiver, this distortion is displayed upon the CRT as an interference pattern of lines and dots. Further, it is understood that such high-frequency radiation may provide radiation to other, neighboring TV receivers, distorting the display of these other TV receivers.
In a further aspect of the TV receiver tuner operation, it is normal to incorporate an automatic fine-tuning circuit (AFT) in connection with the tuner to correct for slight drifting of the tuned signal, and in particular, to develop an error signal to shift the detected signal back toward the correct frequency. In typical television transmission, the picture carrier is set at 45.75 MHz, and as seen in FIG. 5, the output of the IF amplifier at a frequency of 45.75 MHz, is down 6 dB from its output at 44 MHz. Thus, it is seen that in tuning distant stations, the received signal may be otherwise too weak to produce an acceptable picture. In accordance with the objects of this invention, the AFT circuit is detuned slightly to shift the output of the IF amplifier toward the frequency of 44 MHz, whereby the amplitude of the picture carrier signal is increased to enhance the displayed picture.